This invention relates to an improved fuel injector which effectively controls the flow rate of fuel injected into the combustion chamber of an engine.
In most fuel supply systems applicable to internal combustion engines, fuel injectors are used to direct fuel pulses into the engine combustion chamber. A commonly used injector is a closed needle injector which includes a needle assembly having a spring-biased needle valve element positioned adjacent the needle orifices for resisting blow back of exhaust gas into the pumping or metering chamber of the injector while allowing fuel to be injected into the cylinder. The needle valve element also functions to provide a deliberate, abrupt end to fuel injection thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust. The needle valve is positioned in a needle cavity and biased by a needle spring to block fuel flow through the needle orifices. In many fuel systems, when the pressure of the fuel within the needle cavity exceeds the biasing force of the needle spring, the needle valve element moves outwardly to allow fuel to pass through the needle orifices, thus marking the beginning of injection. In another type of system, such as disclosed in U.S. Pat. No. 5,676,114 to Tarr et al., the beginning of injection is controlled by a servo-controlled needle valve element. The assembly includes a control volume positioned adjacent an outer end of the needle valve element, a drain circuit for draining fuel from the control volume to a low pressure drain, and an injection control valve positioned along the drain circuit for controlling the flow of fuel through the drain circuit so as to cause the movement of the needle valve element between open and closed positions. Opening of the injection control valve causes a reduction in the fuel pressure in the control volume resulting in a pressure differential which forces the needle valve open, and closing of the injection control valve causes an increase in the control volume pressure and closing of the needle valve. U.S. Pat. No. 5,463,996 issued to Maley et al. discloses a similar servo-controlled needle valve injector (also referred to as a pilot-actuated needle controlled injector).
Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to meet the ever increasing governmental and regulatory requirements of emissions abatement and increased fuel economy. It is well known that the level of emissions generated by the diesel fuel combustion process can be reduced by decreasing the volume of fuel injected during the initial stage of an injection event while permitting a subsequent unrestricted injection flow rate. As a result, many proposals have been made to provide injection rate control devices in closed needle fuel injector systems. One method of controlling the initial rate of fuel injection is to spill a portion of the fuel to be injected during the injection event. For example, U.S. Pat. No. 5,647,536 to Yen et al. discloses a closed needle injector which includes a spill circuit formed in the needle valve element for spilling injection fuel during the initial portion of an injection event to decrease the quantity of fuel injected during this initial period thus controlling the rate of fuel injection. A subsequent unrestricted injection flow rate is achieved when the needle valve moves into a position blocking the spill flow causing a dramatic increase in the fuel pressure in the needle cavity. However, the needle valve is not servo-controlled and, thus, this needle assembly does not include a control volume for controlling the opening and closing of the needle valve. Moreover, the rate shaping needle assembly does not permit the rate to be selectively varied.
Other rate shaping systems decrease rate of fuel flow during the initial portion of the injection event by, for example, throttling the fuel to the needle orifices. However, many of these systems restrict the flow of fuel throughout the injection event thereby disadvantageously restricting pressure throughout the injection event. This approach is not energy efficient, limits maximum delivery rates and requires increased fuel system operating pressures to maintain maximum desired injection pressures. Other throttling type systems, such as disclosed in FIGS. 6a and 6b of Yen et al., restrict the flow of fuel during the initial portion of an injection event while permitting unrestricted delivery during a later portion. This system uses a single needle valve element which is not servo-controlled.
Although some systems discussed hereinabove create different rate shapes, further improvement is desirable. Therefore, there is need for a servo-controlled fuel injector for providing enhanced selective control over injection timing and duration and variable control of injection rate shaping.
It is an object of the present invention, therefore, to overcome the disadvantages of the prior art and to provide a fuel injector which is capable of effectively and predictably controlling the rate of fuel injection.
It is another object of the present invention to provide a servo-controlled injector capable of effectively controlling the flow rate of fuel injected during each injection event so as to minimize emissions.
It is another object of the present invention to provide a servo-controlled injector assembly capable of shaping the rate of fuel injection which is also simple and inexpensive to manufacture.
It is yet another object of the present invention to provide an injector capable of effectively slowing down the rate of fuel injection during the initial portion of an injection event while subsequently increasing the rate of injection to rapidly achieve a high injection rate.
It is a further object of the present invention to provide an injector for use in a variety of fuel systems, including common rail system, accumulator pump systems and pump-line-needle fuel systems, which effectively controls the rate of injection at each cylinder location.
Still another object of the present invention is to provide a rate shaping injector which is capable of controlling the rate of fuel injection to achieve a more favorable fuel gain response.
Yet another object of the present invention is to provide an injector which permits effective control of fuel injection quantities during small quantity pilot and post injections while permitting injection rate shaping.
Another object of the present invention is to provide a servo-controlled injector which avoids increasing fuel system operating pressures to achieve rate shaping via throttling.
These and other objects of the present invention are achieved by providing a closed nozzle injector assembly for injecting fuel at high pressure into the combustion chamber of an engine, comprising an injector body containing an injector cavity and an injector orifice communicating with one end of the injector cavity to discharge fuel into the combustion chamber wherein the injector body includes a fuel transfer circuit for transferring supply fuel to the injector orifice. The injector also includes a first needle valve element positioned in the injector cavity for controlling fuel flow through the injector orifice and a first valve seat formed on the injector body. The first needle valve element may be movable between a closed position against the first valve seat blocking flow through the injector orifice and an open position permitting flow through the injector orifice. A second needle valve element is also provided which is positioned in the injector cavity and movable between a closed position against a second valve seat blocking fuel flow across the second valve seat and an open position permitting fuel flow across the second valve seat. A throttle passage is formed in the second needle valve element to restrict fuel flow upstream of the injector orifice.
A first needle valve element is preferably an inner needle valve element telescopingly received within a cavity formed in the outer needle valve element. The throttle passage preferably extends through the outer needle valve element. The injector assembly also preferably includes a first control volume positioned adjacent an outer end of the first needle valve element for receiving fuel and a second control volume positioned adjacent an outer end of the second needle valve element for receiving fuel. An injection control valve means is preferably provided for controlling the flow of fuel from the first and the second control volumes. An outer supply cavity may surround the outer needle valve element while an inner supply cavity may be positioned within the outer needle valve element. In this design, the throttle passage fluidically connects the outer supply cavity to the inner supply cavity while being sized to restrict fuel flow to the inner supply cavity.
The injector may further include a drain circuit for draining fuel from the first control volume and the second control volume to a low pressure drain. The injection control valve or valves may further include a first injection control valve positioned along the drain circuit for controlling the flow of fuel through the drain circuit to control the movement of the first needle valve element between the open and closed positions and a second injection control valve positioned along the drain circuit for controlling the flow of fuel through the drain circuit to control the movement of the second needle valve element between the open and closed positions. The first and the second injection control valves may each include an actuator and a reciprocally mounted, selectively movable control valve member. The injector may further include a first biasing spring for biasing the first needle valve element toward the closed position and a second biasing spring for biasing the needle valve element toward the closed position, wherein both the first and the second biasing springs are positioned within the cavity formed in the second needle valve element. The first and the second biasing springs may be positioned in nonoverlapping serial relationship along a longitudinal axis. The actuators for the first and the second needle valve elements may be positioned adjacent one another in side-by-side relationship with respective axes of reciprocation of the control valve members positioned in parallel.
The throttle passage may extend transversely through the second needle valve element from the outer supply cavity to the inner supply cavity. More specifically, the throttle passage may extend substantially perpendicular to a longitudinal axis of the injector body. Also, the injector of the present invention may include a land formed on an inner portion of the second needle valve element immediately adjacent the second valve seat. The land functions as a lift control means positioned within the inner supply cavity for controlling the movement of the second needle valve element from the closed position. The land is exposed to fuel in the inner supply cavity to generate fuel pressure forces that operate to delay the opening of the second needle valve element.